Single base-pair substitution in the beta globin chain produces HbS, which will polymerize when deoxygenated. Eventually the polymerization becomes irreversible and the cell membrane becomes rigid--an irreversibly sickled cell. Although sickle cell trait may produce no anemia or symptons, individuals homozygous for HbS have profound complications: sickle cell crisis with rapid destruction of the sickled cells (as shown in the image--note the marked loss of RBCs), acute chest syndrome, acute sequestration of blood cells, and strokes. HbS is thought to originate in areas endemic for Malaria (ie. Africa, Mediterranean Sea, and India). An infected cell rapidly sickles and is culled out by the spleen, affording protection from this infection.
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